Mercedes admits engine reliability still an "unknown"

Mercedes Formula 1 team boss Toto Wolff admits there are still question marks over the reliability of the Phase 3 engines ahead of the Italian Grand Prix.

Racing Point's Sergio Perez and Williams' Robert Kubica, both of whom suffered failures of their Phase 3 units at Spa, will stick with high mileage Phase 2s in Italy, while Lewis Hamilton, Valtteri Bottas, Lance Stroll and George Russell will use the Phase 3 units.

On Friday in Belgium, all six Mercedes-powered cars were equipped with the latest Phase 3 engines, which were scheduled to last each driver until the end of the season.

However, Perez suffered a failure in FP2 that afternoon, and then Kubica's example failed in his Williams in qualifying on Saturday.

Both men switched to older Phase 2 engines for the rest of the weekend.

After extensive investigation at HPP's Brixworth base this week, some answers have been found, but Wolff admitted that the exact situation was not yet clear.

"We kind of understand a little bit," Wolff told Motorsport.com. "We've had five days of looking at things, and we have indications of what happened. Obviously with these things you can mitigate some of the risks, but it's not completely analysed in the end."

Asked if performance would be compromised in Monza, he said: "That is not yet totally clear. We will know more tomorrow when we have run them. It's still an unknown, we need to run them in FP1."

Regarding the requirement for Perez and Kubica to stick with their old engines, he said: "We wouldn't be able to put the new ones together in time. It's a capacity problem. There were only six. Some spares, but not enough."

Asked about the situation, Hamilton stressed that he had full confidence in HPP and its boss Andy Cowell, and said he had no worries about the company's ability to provide a reliable engine.

"For me, no," he said. "I've spoken to Andy this week, I've had a long journey and experience with this team HPP have done an incredible job, we've been at the forefront of this technology for the past six years. We've been the leaders and everyone's been trying to catch up for years."

Teammate Bottas added: "For the moment what I've heard in the meetings today there are no concerns going forward," said the Finn. "I think everything is understood, and we should be able to run full power. So no concerns for now."